The prior art has made extensive use of dot matrix-liquid crystal display for a variety of purposes. Some of these uses are as a display for a pocket calculator, a general purpose data display device, a television type picture display, an oscillograph, a time display for electronic wrist watches, clocks, and the like. See, for example, U.S. Pat. No. 3,445,827--issued May 20, 1969, to R. W. Keyes for a "Memory Controlled Shift Register Display Device"; U.S. Pat. No. 3,895,372--issued July 15, 1975, to Kaji et al. for a "Quick Response Liquid Crystal Display Device"; and U.S. Pat. No. 4,127,848--issued Nov. 28, 1978to I. A. Shanks for "Liquid Crystal Wave Form Displays."
The problems encountered generally with these prior art systems involve limitations of the LCD display medium, such as limited light, limited viewing angle, the need for complex driving circuitry, and a relatively slow speed of response of the displays employed.
The advantages of a dot matrix format in a LCD is that it can provide more resolution in 17 or 16 segment numerals and alphanumeric fonts than those employed in normal liquid crystal displays. Additionally, a more pleasing, more familiar type font can be used in DM-LCD, and the characters can be made to travel across the face of the LCD panel. Known media for achieving and operating dot matrix displays include incandescent lamps, light emitting diodes (LED), and plasma electroluminescent light sources. Of these, the most familiar is the well-known "Times Square" traveling sign which uses incandescent lamps. The disadvantages of these media are the large space required, the larger power required, and the higher voltages needed to operate the displays.
U.S. Pat. No. 3,493,957--issued Feb. 3, 1970, to W. Brooks, describes a Variable Message Display where the copy appears to move across the display face. In this arrangement, each horizontal row of the display represents a line display controlled by a shift register, and at each shift command each illuminated element (incandescent lamp) appears to move one space to the left. The speed of response, driving power required, size and weight of this device precludes its use in wrist watches and microcomputers for all practical purposes. Further, the device is not capable of producing a traveling image that can be moved horizontally across the display panel and vertically up and down the panel as well.
The Elsimate.RTM. EL-8160 dot matrix pocket calculator manufactured by Sharp Co. includes a dot matrix alphanumeric LCD display in which nine discrete 5.times.7 alphanumeric characters are presented. The characters are separated by a space approximately two dot columns in width and the messages appear to walk across the display, but actually the clearance jump from one space to the next.
All of the above discussed prior art devices require relatively high voltages, high power, and complicated driving schemes, or like the Sharp Elsimate calculator, their displays are extremely limited in format of presentation.
Provision of a dot matrix display which is capable of moving characters smoothly in either of two dimensions, requires a great amount of driving circuitry, or conversely, a microprocessor such as the Intel 8080, with a large program and high operational speeds. As a general rule, the number of operations that a microprocessor performs during a given period of time determines the amount of power consumed by the microprocessor. For products such as wrist watches, where energy is supplied by a battery of limited capacity, this limitation becomes a major importance. For this reason, CMOS circuitry is commonly used for fabricating microprocessors. However, one characteristic of CMOS circuitry is that it is capable of operating at only a small fraction of the speed of NMOS devices such as the Intel 8080, which is a NMOS device.
As an example of the above discussed characteristics, a 7-segment digital wrist watch using a CMOS microprocessor typically operates at a relatively small duty cycle. Periodically, the CMOS microprocessor becomes active, performs a small number of operations, and turns off thereby saving battery power until it receives its next timing turn-on pulse. The more operations the microprocessor performs during each timing pulse, the more current it consumes from the battery. Unfortunately, in order to perform all of the data management and to drive the liquid crystal display for a 10.times.28 dot matrix capable of smoothly moving easily read and pleasing to view characters in two dimensions, requires so many operations of the microprocessor that a CMOS processor operating at normal CMOS speed cannot perform all of the necessary operations within the time periods allowed. If it could, the battery drain would be excessive. Using a NMOS or other faster microprocessor device also would require prohibitive amounts of current and thus their use is precluded in applications where batteries of limited current capacity are employed as the power source. To overcome these problems, the present invention was devised.